The present invention relates to a high-definition television (HDTV) receiver, and more particularly, to an HDTV receiver using a vestigial sideband filter implemented with a finite impulse response (FIR) filter.
FIG. 1 is a block diagram of a conventional HDTV, showing the configuration of HDTV receiver proposed by Grand Alliance.
The conventional HDTV receiver of FIG. 1 comprises a tuner 1, SAW filter 3, IF amplifier 4, local oscillators 2 and 5, demodulator/frequency phase locked loop (FPLL) 6, and an ADC 7.
The operation of the conventional HDTV receiver will be discussed below.
A signal transmitted through an antenna is first and second demodulated in tuner 1. Specifically, the signal input through the antenna is band-pass-filtered in band of 50-800 MHz, and then amplified. The signal of 50-800 MHz output from a BPF/amplifier is multiplied by the frequency signal of 0.97-1.72 GHz output from local oscillator 1b in a mixer 1c for the purpose of first demodulation. Here, local oscillator 1b selects the frequency of an intended channel by a channel tuning signal.
The signal demodulated and output from mixer 1c is band-pass-filtered and amplified to 920 MHz in a BPF/amplifier/AGC 1d. The gain of the amplified signal is automatically controlled under the instruction of a delayed AGC.
In mixer 1e, the signal output from BPF/amplifier/AGC 1d is multiplied by the frequency signal output from local oscillator 2 and whose central frequency is 876 MHz. The local oscillator is a VCO in which 876 MHz is the central frequency. The multiplied signal is output, with its intermediate frequency being 44 MHz. The output signal of mixer 1e is gain-controlled and amplified in IF amplifier 1f. The signal output from IF amplifier 1f is VSB-filtered in SAW filter 3.
Speaking of the transmission/reception terminals of the HDTV in which a signal is transmitted in VSB, a transmission function for the overall transmission/reception must be raised-cosine in a transmission band in which a pilot signal is present, in order not to produce inter-symbol interference.
On the transmission terminal of HDTV, a square root raised cosine (SRC) VSB filter is used. Accordingly, the SRC filter must be also used on the reception terminal of HDTV.
FIG. 2 is a frequency characteristic diagram of an SRC-type VSB filter.
Referring to FIG. 2, an SRC-type VSB filter becomes a transmission channel free from inter-symbol interference because the SRC of the transmission terminal and the SRC of the reception terminal are multiplied to become a RC filter.
In other words, the RC filter is made by multiplying the transmission filter by the reception filter. Here, the transmission filter and the reception filter are the same, becoming the SRC filter.
This can be given in the following equation.
RC filter=transmission filter.times.reception filter transmission filter=reception filter=RC filter=SRC filter
The conventional VSB filter for HDTV is realized with a SAW filter 3. The frequency characteristic of SAW filter 3 is as shown in FIG. 2.
The signal VSB-filtered through SAW filter 3 receives an AGC signal from IF amplifier 4 so as to be gain-controlled automatically. The gain-controlled signal is demodulated to a baseband in demodulator/FPLL 6 so that its frequency and phase are locked.
Here, the FPLL forms two loops. One loop formed with a mixer 6a, AFC LPF 6d, amplifier limiter 6e, APC LPF 6f, and local oscillator 2 is to lock frequency. The other loop formed with a mixer 6c, APC LPF 6f and local oscillator 2 is to lock phase when the frequency is locked and only "1" is output from amplifier limiter 6e. Here, local oscillator 5 generates a single frequency of 46.69 MHz with a reference oscillator. The single frequency is output to mixer 6c and a phase converter 6b for performing 900 phase conversion.
The signal passing through demodulator/FPLL 6 is digitally converted in ADC 7 at 10.76 MHz of the symbol rate.
In the conventional HDTV receiver, since the AFC signal output from demodulator/FPLL 6 controls local oscillator 2, the loop is elongated to make the system unstable.
In addition, since the VSB filter of the conventional HDTV is constructed with a SAW filter, roll-off factor .alpha., the ratio of transmission band to pass band, is given as follows. ##EQU1##
As shown in this equation, the factor is so small as to require large number of taps, complicating the configuration of the SAW filter and reducing economical efficiency.